The invention relates to an integrated optical channel having superior performance for transmitting optical signals between at least one source and at least one destination when compared with the prior art, an improved method of transmitting optical signals between a source and one or more destinations, and an optical switch and method for switching optical signals, in which said integrated optical channel confers the advantages of transmission of signals with reduced loss of power and enhanced retention of signal quality when compared with the prior art.
The performance of fiber optic communication systems is dependent on the strength of a signal that can be transmitted along a length of optical fiber in an optical communication channel and retention of the quality of the signal during transmission. A signal that has lost a portion of its strength during transmission must be boosted to recover that strength before further transmission, or else the signal will be too weak to be detected or understood after transmission. Similarly, the quality of the signal must be retained if it is to be clearly understood. Losses in signal strength and quality can occur when a signal is communicated between a source and a destination along a connecting optical channel. A connecting optical channel can include one or more devices such as an optical switch, a monitor, a tap, an attenuator and a filter. Presently, transmission of an optical signal through a device such as an optical switch can cause a significant loss in signal strength, especially when an optical signal emitted from one optical fiber is redirected to another optical fiber by transmission via several reflectors or refractors between the optical fibers. Consequently, there is a need to improve capability to transmit a signal through optical channels having one or more reflective and/or refractive devices therein.
When an optical device that is a refractor, such as a lens, or a reflector, such as a mirror, is situated within an optical channel, there is an insertion loss of strength of an optical signal that is transmitted via that device. A state of the art optical channel typically contains two lenses for collimating and directing an optical signal transmitted in free space through that optical channel, for example from a first optical fiber to a second optical fiber. Losses in strength of an optical signal can also arise, for example, from dispersion of the collimated beam.
What is required is an improved method and apparatus for transmitting fiber optic signals between a source and a destination. What is also required is an improved method and apparatus for switching and/or monitoring optical communications between a source and a destination.
According to the PRIOR ART, in an optical channel connecting an optic fiber with a light receiving device, a light signal that is emitted from a distal end of an optic fiber is collimated in order to be transmitted along an optical channel with a high degree of retention of signal strength and quality. Apparatus and methods for collimating a signal beam are described in, for example, U.S. Pat. No. 6,198,858, issued to Pan et al. in 2001, U.S. Pat. No. 6,246,812, issued to Liu and Chang in 2001, and U.S. Pat. No. 6,263,133, issued to Hamm in 2001. We have found that there is no need to use any of these complex means for collimating an optical signal beam when using the apparatus and method of the present invention, and so the present invention offers advantages of simplicity of construction and operation, and hence reduced cost.
We have found that, by aligning several lenses in a regular pattern along an optical axis of an optical channel, insertion losses can be greatly reduced and a signal beam can be transmitted along said optical channel with high retention of optical signal strength and quality.
According to one aspect of the present invention there is provided a first embodiment of an apparatus providing a body having at least one first optical channel and an N-fold first plurality of lenses, the lenses having substantially similar sizes and optical properties. The N-fold plurality of lenses are spaced at regular intervals L1 along a first optical axis extending in a straight line between the first end and the second end of the optical channel. A first lens is distanced by L2 from the first distal end of a source, such as a first optical fiber, situated at a first end of the first optical channel. L2 is about one-half of L1. According to theoretical calculations, ideally L2 is exactly one-half of L1 for a lens having perfect optical properties. When the first optical channel is a through channel, a second lens is distanced by L2 from the second distal end of a destination such as a second optical fiber situated at a second end of the first optical channel. Values of L1 and L2 are selected so that a light signal transmitted at one of the first end and the second end of the first optical channel is refocused by each of the succession of lenses to form a regular (Nxe2x88x921-fold plurality of waists along the first optical axis, one of waists being situated between each pair of the plurality of lenses, and finally is focused at the other of the first and the second end.
According to another aspect of the present invention there is provided a second embodiment of the apparatus which is similar to the first embodiment except that the spacings between successive ones of the N-fold plurality of lenses in the first optical channel comprise a regular repeating pattern of two different spacings Lxe2x80x2 and Lxe2x80x3 along the first optical axis. The pattern is such that a light signal transmitted at one of the first end and the second end of the first optical channel is refocused by each of the succession of lenses and finally focused at the other of the first end and the second end.
According to yet another aspect of the present invention there is provided a third embodiment of the apparatus which is similar to the first and the second embodiments except that the N-fold plurality of lenses in the first optical channel comprises more than one sets of lenses, the lenses within any one set having closely similar properties. The lenses are aligned in a regular pattern along the optical axis of the first optical channel, the pattern being such that a light signal transmitted at one of the first end and the second end of the first optical channel is refocused by each of the succession of lenses to form a (Nxe2x88x921)-fold sequence of waists, one of which is between each pair of the plurality of lenses, and finally is focused at the other of the first end and the second end.
According to another aspect of the present invention there is provided a method for transmitting fiber optic signals between a source and at least one destination in which at least one of the first, second and third embodiments of the apparatus is provided as described above. A transmitting end of the source is at the first distal end of a first optical fiber situated at the first end of the first optical channel. A receiving end of the destination is at the second distal end of a second optical fiber of the first optical channel. The first optical fiber and the second optical fiber are in optical communication through the first optical channel. The positions of the lenses are selected so that a light signal emitted from the first distal end of the first optical fiber is continuously refocused by each of the succession of lenses and is then focused at the second distal end of the second optical fiber. The light signal transmitted via this arrangement of lenses has a high retention of signal quality and a low insertion loss between the source and the destination.
Although beneficial results may be obtained through the use of the apparatus for either transmitting an optical signal or switching an optical signal, as described above, it has been found that loss of strength and loss of coherence of the signal beam through dispersion between lenses are both minimized when first length L1 has a value that is not greater than four times the focal length of any one of the lenses.
Although beneficial results may be obtained through the use of the apparatus, as described above, it has been found that, when each apparatus described above is to be used for fiber optical communications, said apparatus preferably is manufactured by a process comprising a combination of micromachining and/or etching the shape of the movable portions and the base from a monolithic wafer. Manufacturing the apparatus from a monolithic wafer conveys several advantages, especially for the manufacture of the micro-optical path switches required for switching an optical signal between one optical fiber and another optical fiber according to the method of the present invention. One advantage is that all the components so manufactured can be made from a single substrate, and so can be very accurately situated relative to each other. Thus there is no need to assemble the movable portions and the base to construct the apparatus. Another advantage is that several of the apparatus can be made from a single monolithic wafer. Yet another advantage is that an array of plurals of the apparatus, and when necessary ancillary apparatus, can be manufactured simultaneously from a single wafer. It has been found that the apparatus can be manufactured by micromachining and/or etching a monolithic wafer comprising, as a non-limiting example, a first layer that is silicon, a second layer that is silicon dioxide and a third layer that is again silicon. When the movable portion is a portion of the first layer and the base includes the third layer, the portion of the second layer that is situated between the movable portion and the base can be removed by etching the silicon dioxide, thereby allowing the movable portion to move relative to the base. It will be recognized by one skilled in the art that monolithic wafers other than that used as an example above can be used, including combinations of layers of silicon and silicon nitride, and combinations of elements other than silicon and compounds other than compounds of silicon.
Beneficial results may be obtained through the use of the apparatus, as described above, when the lenses are any one of conventional refractive elements. It has been found that choosing ball lenses as the lenses of the invention provides convenience in implementing the invention. In the particular case when the lenses are ball lenses, it has been found that beneficial results are obtained when the first lens is spaced from the first distal end of the first optical fiber by a second length L2 that is about 5% less than one-half of first length L1. When L2 is about 5% less than one-half the length of L1, an improvement in the quality of the light signal transmitted along the first optical channel is achieved by reduction of the impact of spherical aberration arising from the shape of the ball lenses. A similar effect has also been found for another application of ball lenses, as described by Upton and Koshel in xe2x80x9cModeling coherent propagation aids accurate couplingxe2x80x9d, in the June, 2001 issue of WDM SOLUTIONS, published by PennWell Corporation.
Although beneficial results may be obtained through the use of the apparatus, as described above, wherein the lenses are ball lenses, even more beneficial results are obtained when the ball lenses have an anti-reflective coating to reduce scattering of light.
The principals of application of ball lenses for fiber optical communications are described by Kennedy in xe2x80x9cUnderstanding Ball Lensesxe2x80x9d, an article at the commercial web site http://www.edmundoptics.com/techsupport/DisplayArticle.cfm?articleid=245. Ball lenses having full-surface anti-reflective coating are described in the commercial web site http://ourworld.compuserve.com/homepages/awi_industries/Optic_ballLens.htm.